Many abnormal medical conditions in humans and other mammals have been associated with disease and other aberrations along the lining or walls that define several different body spaces. In order to treat such abnormal conditions of the body spaces, medical device technologies adapted for delivering various therapies to the body spaces using the least invasive means possible.
As used herein, the term “body space,” including derivatives thereof, is intended to mean any cavity within the body which is defined at least in part by a tissue wall. For example, the cardiac chambers, the uterus, the regions of the gastrointestinal tract, and the arterial or venous vessels are all considered illustrative examples of body spaces within the intended meaning.
The term “vessel,” including derivatives thereof, is herein intended to mean any body space which is circumscribed along a length by a tubular tissue wall and which terminates at each of two ends in at least one opening that communicates externally of the body space. For example, the large and small intestines, the vas deferens, the trachea, and the fallopian tubes are all illustrative examples of vessels within the intended meaning. Blood vessels are also herein considered vessels, including regions of the vascular tree between their branch points. More particularly, the pulmonary veins are vessels within the intended meaning, including the region of the pulmonary veins between the branched portions of their ostia along a left ventricle wall, although the wall tissue defining the ostia typically presents uniquely tapered lumenal shapes.
One means of treating body spaces in a minimally invasive manner is through the use of catheters to reach internal organs and vessels within a body space. Electrode or electrophysiology (EP) catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity. In use, the electrode catheter is inserted into a major vein or artery, e.g., the femoral artery, and then guided into the chamber of the heart that is of concern in order to perform an ablation procedure.
A typical ablation procedure involves the insertion of a catheter having a tip electrode at its distal end into a heart chamber. A reference electrode is provided, generally taped to the skin of the patient. RF (radio frequency) current is applied to the tip electrode, and current flows through the media that surrounds it, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated sufficiently to cause cellular destruction in the cardiac tissue resulting in formation of a lesion within the cardiac tissue that is electrically non-conductive.
Tips of EP catheters for use in an ablation procedure typically are made from a platinum-iridium alloy. Although this material is not as thermally conductive as other materials such as copper or aluminum, the platinum-iridium alloy is more biocompatible than copper or aluminum. The platinum-iridium alloy, however, is costly to manufacture. Therefore, it would be desirable to provide an EP catheter having a tip electrode that is more thermally conductive than present EP tip electrodes but which would cost less to manufacture.
Additionally, it is difficult to solder leads to platinum-iridium tip electrodes for catheters. It would be preferable to have an EP tip electrode that would provide a platform for easier soldering of the leads.
Using pure gold would provide a tip electrode having high conductivity but would be prohibitively expensive. Therefore, it would be preferable to have an EP tip electrode that provides high-conductivity but at a lower cost than pure gold.
In RF ablation the tissue is heated sufficiently to cause cellular destruction in the cardiac tissue resulting in formation of a lesion within the cardiac tissue that is electrically non-conductive. During this process, heating of the electrode also occurs as a result of conduction from the heated tissue to the electrode itself. If the electrode temperature becomes sufficiently high, possibly above 60 degrees centigrade, a thin transparent coating of dehydrated blood protein can form on the surface of the electrode. If the temperature continues to rise, this dehydrated layer can become progressively thicker resulting in blood coagulation on the electrode surface. Because dehydrated biological material has a higher electrical resistance than endocardial tissue, impedance to the flow of electrical energy into the tissue also increases. If the impedance increases sufficiently, an impedance rise occurs and the catheter must be removed from the body and the tip electrode cleaned.
One method used to reduce the negative affects of heating is to irrigate the ablation electrode, e.g., with physiologic saline at room temperature, to actively cool the ablation electrode instead of relying on the more passive physiological cooling of the blood. Because the strength of the RF current is no longer limited by the interface temperature, current can be increased. This results in lesions that tend to be larger and more spherical, usually measuring about 10 to 12 mm. In addition to irrigation flow during ablation, a maintenance flow, typically at a lower flow rate, is required throughout the procedure to prevent backflow of blood flow into the coolant passages. Thus, it is necessary to provide for catheters that provide lumens for irrigation to the cool the tissue. Where irrigation is not possible it would be desirable to have an EP catheter that would act as a heat sink to cool tissue during ablation.
Another issue for EP catheters arises when they are used in RMT systems. In remote magnetic technology (RMT) systems, magnets external to the patient are used to produce magnetic fields in the patient that can guide a catheter such as an RF catheter for ablation. Catheters used for this purpose must have a high degree of flexibility so that the magnetic fields can properly guide the device through the tortuous anatomy of the patient. EP catheters used in RMT systems must also have reduced thermal reaction time and increased thermal accuracy. EP catheters for RMT systems are usually formed with a thin-walled shell that leaves room for a magnet used to navigate the tip of the catheter. This magnet causes the thermal conductivity of the tip of the RMT catheter to be much lower than it should ideally be. Therefore, it would be desirable to have an RMT catheter that increases the thermal conductivity of the shell material in order to compensate for the lack of thermal conductivity of the necessary magnets therein.
Additionally, as EP catheters become more complex, it would be desirable to have a tip electrode that would permit various sensors or other electronics to be housed while having similar thermal characteristics to existing EP catheter tips electrodes.
Also, in a feedback system in which tissue temperature changes are used to control the application of power to the ablation element it would be desirable to have an EP catheter tip electrode with a faster thermal reaction time in order to allow control with greater precision.